I know that I can launch Screen as a serial port terminal from scratch. What I want to do is to create a new window inside an already running screen session as a serial terminal. so that I don't need to create a window and run minicom in it.
Type this on a command line within a screen session.
For a built in serial port.
screen -X screen /dev/ttyS0
For a usb-to-serial port adapter.
screen -X screen /dev/ttyUSB0
Note: '-X' is a capital X.